1. Field of the Invention
The present invention relates to a multicast network monitoring method and a multicast network system to which the same is applied.
2. Description of the Related Art
A multicast network means a network that performs a so-called push type operation of copying and distributing a packet to other terminals in the same group when one terminal in a group including a plurality of terminals transmits the packet to the network.
FIG. 1 shows a configuration example of a multicast network system with a multicast network function, which is used for providing services such as IPTV or remote lectures. In FIG. 1, a multicast network 1 includes a plurality of relay routers RT.
When one packet is input to the relay router RT of the multicast network 1, the packet is copied and output to each user terminal STU. Therefore, content from a distribution server 2 for content, etc., can be distributed to a plurality of the user terminals STU at the same time. To realize such a multicast network, two protocols operate.
One protocol is a membership management protocol used when a terminal notifies the network of participation in a group and the other protocol is a multicast routing protocol for determining a distribution path of a packet in the network.
The former membership management protocol is represented by IGMP (Internet Group Multicast Protocol) [RFC1112] (Note 1), [RFC2236] (Note 2), and the MLD (Multicast Listener Discovery) [RFC2710] (Note 3). The IGMP is used in the IPv4 network and the MLD is used in the IPv6 network.
With regard to the operation, the protocol is used when a terminal notifies an edge router of participation in a group. Specifically, the terminal notifies the edge router of participation in a multicast address (=group address) of class D. When a packet sent to the specified group address arrives, the edge router copies and transmits the packet to the terminal that has sent the notification (accurately, an interface connected with the terminal).
As a general rule, one terminal can belong to a plurality of groups.
The latter multicast routing protocol is a protocol used when a terminal notifies the edge router of participation in a group and the traffic to the corresponding group address does not arrive. The protocol is represented by PIM-SM (Protocol Independent Multicast-Sparse Mode) [RFC2362] (Note 4), and PIM-SSM (PIM-Source Specific Multicast) [RFC3569] (Note 5).
With regard to the operation, the edge router notifies an upstream next-hop router in a shortest path to a predefined router (representative router or transmission source router) of a request for participation in the group to request transfer of the traffic to the relevant group. If the traffic does not arrive in the upstream router, this operation is repeatedly performed until reaching an upstream router that can distribute the traffic.
(Note 1) [RFC1112] “Internet Group Multicast Protocol (IGMP)”, IETF, RFC1112
(Note 2) [RFC2236] “Internet Group Multicast Protocol, Version 2”, IETF, RFC2236
(Note 3) [RFC2362] “Protocol Independent Multicast-Sparse Mode (PIM-SM)”, IETF, RFC2262
(Note 4) [RFC2716] “Multicast Listener Discovery (MLD) for IPv6”, IETF, RFC2716
(Note 5) [RFC3569] “An Overview of Source-Specific Multicast (SSM)”, IETF, RFC3569
In such a multicast network system, since importance is attached to a real time property of the simultaneous distribution in the function of the distribution server 2, the transmission is performed using RTP (Real-Time Transport protocol)/UDP (User Datagram protocol), which uses no transmission confirmation.
Therefore, if a failure occurs in the multicast network 1, a sender, especially, a content provider cannot know the occurrence of the failure. Therefore, a distribution monitoring server 3 is disposed to monitor the network.
Several conventional arts are applied to such network monitoring of the monitoring server 3. The network monitoring will be described as follows.
(1) Communication Monitoring with ICMP (Internet Control Message Protocol) Ping Mode for Each User Terminal STU
The ping mode is a mode when the monitoring server 3 sends the ICMP Ping to each terminal STU and receives responses to check connectivity of the network 1.
The monitoring server 3 sends the ping to each terminal STU in order. Therefore, if a large number of terminals exist, intervals of sending the ping to one terminal are increased due to a server load, and it is problematic that detection of a failure is delayed.
Since the ICMP Ping is not distributed with the multicast mechanism (e.g., the ping may arrive at the terminal STU via a different path), it is problematic that the ICMP Ping is essentially inadequate for the communication monitoring of the multicast network 1.
(2) Multicast-Base Communication Monitoring
i. ACK Mode: The monitoring server 3 multicasts acknowledgment request packets to the terminals STU and checks presence of an acknowledgment from each terminal STU to detect occurrence of abnormality.
In this mode, as the number of the terminals STU increases, more packets arrive at the monitoring server 3 and, eventually, the monitoring server 3 will not be able to process the packets (this is referred to as ACK Implosion, which is a well-known behavior).
ii. ACK Aggregation Mode: FIG. 2 is a diagram describing an ACK aggregation mode and, as shown in FIG. 2, a plurality of ACK aggregation servers is disposed in a network (In the example of FIG. 2, terminals STU1, STU2, STU3 carry out an aggregation function). Instead of directly returning ACKs to the monitoring server 3, the terminals notify the aggregation servers STU1, STU2, STU3 of the ACKs. By checking the ACKs at the servers, the implosion can be prevented in the number of the acknowledgment packets to the monitoring server 3 in the case of the above ACK mode.
However, in this ACK aggregation mode, if the terminals increase, more aggregation servers are needed, and it is problematic that implosion occurs in time-out notifications from aggregation servers. If a plurality of stages is formed by the aggregation servers to avoid the implosion of the time-out notifications, the time-out must be waited at each stage and it takes time to detect a failure.
A known background art includes a technology of performing multicast communication that a first computer transmits a request message to a second computer to receive acknowledgment messages in a network computer system with a plurality of computers connected through a network (Japanese Patent Application Laid-Open Publication No. 1999-110365). Although recovery from a failure of a computer is shown in Japanese Patent Application Laid-Open Publication No. 1999-110365, detection of a failure in the network is not mentioned.
Although a plurality of monitoring modes are proposed as described above, it is difficult in any case to detect network communication disconnection that may affect a multiplicity of users in a large-scale multicast network.